1. Field of the Invention
The present invention relates to a method for molding a steering wheel for use in a vehicle. More particularly, the present invention relates to a method for molding a steering wheel free of a core embedded in its circumferential ring.
2. Description of Related Art
There has been proposed a steering wheel having a core embedded in a resin. Due to the core, the steering wheel of this type is advantageous in that it is durable against deformation or the like.
It has been desired for this technology to reduce the cost and the weight of the steering wheel. In order to meet this desire, a steering wheel has been developed having no core in its spoke or in its ring. This technology is disclosed in Japanese Patent Laid-Open No. 57-33067, for example. According to this technology, the coreless steering wheel is inferior in respect of rigidity as compared to the steering wheel having a core. However, the cost and weight of the coreless steering wheel is less than one having a core.
Incidentally, the steering wheel of this type is generally constructed to have its inside filled with a resin (in a solid state). Specifically, at the time of molding of the steering wheel, a mold having a predetermined shape has its cavity filled with a plastic resin. After this resin has solidified, the steering wheel is obtained by extracting the solidified resin.
If, however, the solidification of the resin is insufficient at a cooling step of the molding process, the resin is liable to deform. Therefore, the resin cooling time period has to be extended so as to set the resin completely.
In case the steering wheel having the core is to be molded of a polypropylene resin, for example, the resin cooling time period required is about twenty to forty seconds. However, when a steering wheel having no core is to be molded of a polypropylene resin, the resin cooling time period required is as long as about 90 to 120 seconds. At the time of molding of the steering wheel having no core, therefore, a longer cooling time period is required in the molding method of the prior art, which decreases the working efficiency.